A variety of temperature responsive current interrupters have thus far been proposed and put into practical use for the protection of electric appliances from being overheated by ambient temperatures. Such devices are largely categorized as those of the types which use electrically conductive, low-melting-point metals or alloys as the temperature responsive fusible elements and those of the types which use electrically nonconductive, thermally fusible temperature responsive elements. A typical example of the known temperature responsive current interrupters using low-melting-point metals or alloys is the device in which two current conductors are normally connected by a fusible elements of a low-melting-point alloy and are urged to be disconnected from each other by suitable biasing means such as a weight or a preloaded spring. The mechanical and accordingly electrical connection between the two current conductors is broken by the action of such biasing means when the fusible element between the conductors is caused to melt by heat exceeding a predetermined temperature. A representative example of temperature responsive current interrupters or fuses of this nature is disclosed in U.S. Pat. No. 3,639,874 in which the biasing means acting on the current conductors are constituted by preloaded springs. Current interrupters thus using fusible elements of low-melting-point metals or alloys are advantageous in that the fusible elements interconnecting the current conductors are electrically conductive and are per se operable to provide electrical connection between the current conductors without aid of any extra members mechanically connecting the conductors. Such current interrupters are, however, not fully acceptable because of the difficulty in accurately controlling the melting points of the fusible elements of the individual interrupters so that the melting points of the fusible elements are liable to vary from one interrupter to another or from one lot of interrupters to another. Because, moreover, of the fact that the fusible elements used as the electric connectors are subjected to oxidizing effects due to the currents which flow therethrough during use of the current interrupters, the melting points of the fusible elements of low-melting-point metals or alloys are inevitably subject to change with time.
These problems encountered in temperature responsive current interrupters of the types using fusible temperature responsive elements of low-melting-point metals or alloys are alleviated or practically eliminated in current interrupters of the types which use electrically non-conductive, thermally fusible temperature responsive elements because the melting points of such elements can be easily and accurately controlled during production of the interrupters on a large-scale commercial basis and are maintained substantially unchanged throughout the use of the interrupters since the temperature responsive elements per se are not used as electric connectors for the current conductors of the interrupters and are therefore free from oxidizing effects. However, because, of the fact that the temperature responsive fusible elements of electrically non-conductive properties are not operable as means for electrically connecting conductors, extra elements are required for providing electrical connection between the conductors. Provision of such extra mechanical elements not only adds to the total number of the component parts and accordingly the production cost of a current interrupter but raises a problem in controlling the performance characteristics of the interrupter due to the sliding frictions produced between the mechanical elements or connectors which are moved from the positions providing electrical connection between the current conductors to positions interrupting such connection when the temperature responsive fusible elements are melted away by unusually high ambient temperatures, as will be discussed in more detail as the description proceeds. The present invention contemplates elimination of these drawbacks which have been inherent in prior-art temperature responsive current interrupters using electrically conductive or non-conductive, thermally fusible temperature responsive elements.
It is therefore, an important object of the present invention or provide an improved temperature responsive current interrupter or fuse featuring, inter alia, ease and accuracy in controlling the melting point of a temperature responsive fusible element forming part of the current interrupter during production of the device.
It is another important object of the present invention to provide an improved temperature responsive current interrupter or fuse in which the temperature responsive fusible element is free from oxidizing effect resulting from the flow of electric current through the current interrupter during use of the device and in which the melting point of the fusible element can be maintained substantially constant throughout the period of time for which the interrupter is in use.
It is still another important object of the present invention to provide an improved temperature responsive current interrupter which includes no such mechanical element as to be subjected to sliding friction when the current interrupter is caused to break the electrical connection between the current conductors of the device and which is capable of providing reliability in cutting off the flow of current therethrough in response to a rise in the temperature of the ambient heat to a predetermined threshold level.
It is still another important object of the present invention to provide an improved temperature responsive current interrupter which is simple in construction and which is accordingly easy and economical to manufacture on a large-scale commercial basis.
Yet, it is another important object of the present invention to provide an improved temperature responsive current interrupter in which both electrically conductive and non-conductive temperature responsive fusible elements are used in combination so that the advantages of prior-art temperature responsive current interrupters of both of the types using conductive and non-conductive fusible elements are exploited in simple configuration.